Medical endoprostheses such as stents can be placed within the body to perform a function such as maintaining open a body lumen, for example, a passageway occluded by a tumor or a blood vessel restricted by plaque. Other endoprostheses such as stent-grafts, or covered stents, can be used to substitute for or reinforce a lumen, such as the aorta or other blood vessels that have been weakened, e.g., by an aneurysm.
Endoprostheses can be delivered inside the body by a catheter that supports an endoprosthesis in a compacted or reduced-size form as the endoprosthesis is transported to a desired site. The size is particularly small when a percutaneous insertion technique is employed. Upon reaching the site, the endoprosthesis is expanded, for example, so that it can contact the walls of the lumen.
The expansion mechanism may include forcing the endoprosthesis to expand radially. For example, the expansion mechanism can include the catheter carrying a balloon, which carries the endoprosthesis. The balloon can be inflated so as to deform and fix the expanded endoprosthesis at a predetermined position in contact with the lumen wall. The balloon can then be deflated, and the catheter removed.
Balloon expandable endoprostheses can provide relatively good stiffness and relatively accurate placement. The force provided by the balloon also provides the endoprostheses with good vessel expansion and secure fixation. However, balloon expandable endoprostheses can have relatively low crush resistance, relatively low flexibility, and, in some cases, uncertain attachment to the expansion mechanism, e.g., the balloon.
In another delivery technique, the endoprosthesis is formed of an elastic material that can be reversibly compacted and expanded. During introduction into the body, the endoprosthesis is restrained in a compacted condition. Upon reaching the desired implantation site, the restraint is removed, for example, by retracting a restraining device such as an outer sheath, enabling the endoprosthesis to self-expand by its own internal elastic restoring force.
Self-expandable endoprostheses can provide good flexibility and a persistent radial expansion force, which can provide good crush resistance. However, self-expandable prostheses can have relatively inconsistent or poor placement accuracy. In some cases, a separate balloon dilation procedure is necessary to ensure proper expansion and contact. Furthermore, the restraining device can increase the cross sectional profile of the endoprosthesis, which can limit the number of implantation sites where the endoprosthesis can be used.